Over the years, motorcycles have increasingly gained in popularity. This is especially true with respect to large touring bikes. Correspondingly, improvements in motorcycle technology relating to improving handling and stability of the motorcycle during its ride has been direly sought.
A critical component of stability in operation of the motorcycle is the front-end assembly comprising a two-pronged fork having two rods which telescopically fit within, respectively, a pair of bottom case prongs, which is attached to the front wheel and which is also connected to the steering mechanism of the motorcycle.
Specifically, it has been discovered by the present inventor that problems are encountered with the bottom case prongs of the front end of motorcycles during turning and braking, as these conditions present unbalanced loads on the front fork assembly, causing the same to twist and deflect, allowing the front wheel to take positions and angles other than as commanded by the handlebar position. The resulting wheel motion or softness in the steering can lead to results ranging from an annoyance to total loss of control of the vehicle.
In the case of braking, the recent large touring motorcycles have disc brakes on both the left and right sides of the front wheel. However, these motorcycles also have what is referred to as integrated braking, whereby actuation of the "rear" brake control results in simultaneous actuation of the brake mechanisms at the right side of the rear wheel and at the left of the front wheel (rather than both sides of the front wheel). Thus, all of the braking torque for the front wheel must be absorbed by one side of the fork assembly only. Older cycles only have a brake mechanism on one side of the front, so that front braking is always unbalanced. Since the braking torque is a torque tending to rotate that side of the fork assembly rearwardly, the resulting deflection of the fork assembly will result in the front wheel turning to the left, a condition which, depending upon speed and braking rate, must be quickly compensated for. Further of course, the compensation must be proper in the sense that over compensation can lead to aggravation of the problem rather than the solving of the same. By way of example, if the rider turns the handlebars to the right to try to quickly compensate for the accumulated left turn, and at the same time releases the brake to gain control, the front wheel may turn to the right further than it originally deflected to the left, etc. Consequently, the front wheel spin axis may oscillate in a horizontal plane depending upon the amount of braking, the rider response and the softness of the front fork assembly.
Also, depending upon speed, the front wheel may have a substantial gyroscopic effect. Since the front fork assembly is a spring mounted assembly, vertical motion of the front wheel axis is reasonably unrestrained, so that again, depending upon the rigidity of the front fork assembly, the wheel spin axis may alternately dip to the left and right. The resulting motion of such a rotating body is referred to nutation, which motion will have a frequency dependent upon the speed of the vehicle, and at certain speeds may be grossly aggravated rather than compensated for by the response time of the rider in controlling the handlebar position. Of course this all occurs at a most inopportune time, as braking is normally done when a need to do so arises, and generally cannot be significantly delayed, such as might be required to regain full control of the motorcycle.
Another condition where the softness of a front fork assembly is particularly evident and disadvantageous is in turning, particularly low speed turns, as one might make in traffic to enter a driveway, etc. Here the forces on the front wheel may vary considerably, dependent on how far the front wheel is turned, how fast the motorcycle is going, whether power or braking is being applied, etc. With the front wheel turned, the wheel is also inclined substantially with respect to the vertical (even with the machine held vertically as when stopped), so that the front wheel is standing on the edge thereof, tending to deflect the fork assembly accordingly. If on the other hand, power is being applied during the turn, the front fork assembly will tend to deflect in an opposite direction, with braking as herein before described affecting the extent of turn of the front wheel. These various factors affect not only the turn of the front wheel, but also the location of the contact of the front wheel with the ground, affecting the balance of the vehicle when the vehicle is not very stable to start with because of the slow speed. Accordingly, a large number of spills and/or near spills occur on the type of motorcycle described during low speed turns, again because of the softness of the front fork assembly of the motorcycles and the various and variable forces thereon caused by various turning conditions.
Further, there is an additional situation which causes instability while driving, namely, the grooving of highways to improve traction between tires and the highway surface particularly in rainy weather. These rain grooves, now quite common, generally are aligned with the direction of vehicle travel down the highway, though only approximately, as the grooves are cut without any real concern in the accuracy of the alignment thereof. Further of course, the vehicle itself will not necessarily be headed straight down the road, whether a motorcycle, car or otherwise, and will generally change positions within a lane and/or change lanes from time to time as traffic conditions suggest. Consequently, since a vehicle's tires will only be approximately aligned with the direction of the rain grooves, depending upon conditions, the rain grooves may react with the vehicle's tires to prevent substantial and time varying side loads thereon. In the case of motorcycles of the type described, these side loads themselves can excite front wheel nutation and cause steering wander which can lead to a single vehicle straight and dry road crash.
A component of prior art front-end fork assemblies is a fork bridge which is adapted to fit between the two prongs of the motorcycle fork above the "slider" unit thereof. However, this prior art fork bridge does not address or recognize the source of the aforementioned problems in that independent flexing of the lower case fork prongs is not prevented.
More specifically, prior art fork bridges have a hollow and thin contour and are typically comprised of cast aluminum, not precision machined, provide minimal contact with the lower case fork prongs and do not serve to partially capture the lower case fork prongs, thereby failing to prevent independent flexing of the lower case fork prongs. Such prior art fork bridges allow the fork prongs to flex and turn independent of one another, which the forks were not designed to do. Since sideload pressure exerted on one or the other fork prong during operation of the motorcycle (i.e. during driving) can, depending on load, speed and degree of curve, braking, torque, etc., reach up to 30,000 pounds per square inch, it will be appreciated that a motorcycle front-end fork assembly must be able to adequately resist twisting forces of great magnitude.
Since the inability of the prior art fork bridge to control the twisting of the forks causes a loss of control during steering and braking, thereby often resulting in a complete loss of control of the motorcycle and resulting injury to the driver, proper stabilization of motorcycles is a critical and unsolved problem.